© Copyright 2018 Orlando Alves da Silva
Dysfunction of Egocentric Spatial Location
How does the brain of an observer know where surrounding objects are in relation to the observer? How does the brain know where the eyes are in relation to the rest of the body?
The exact location of an object in relation to an observer is given by the perception of the position of the eyes in relation to the rest of the observer’s body, and by the perception of the position of the eyes in relation to the object. It is not the retina that informs the brain about the exact location of the objects that surround the observer, in relation to the observer.
The image of the object is projected, independently of its location in relation to the observer, to a specific visual area in the occipital lobe. This visual cortical area is the only area in the brain to receive this specific information about the object; the brain knows where the eyes are because of the perception of the difference in tonus between the oculomotor muscles. When looking straight ahead, there is a tonic equivalence between the lateral rectus and the internal rectus in each eye. When looking sideways, one of these muscles has higher tonus than the other, as one is contracted and the other is relaxed.
It is the perception of higher or lower tonus that indicates to the brain the location of the target object. By changing the muscular tonus of oculomotor muscles, the egocentric spatial localisation can be changed. The correct perception of the relative tonicity of the oculomotor muscles is crucial - if there is a dysfunction of proprioception, this perception is altered.
The Eye-Hand Test was developed by Orlando Alves da Silva to assess egocentric spatial location by evaluating the accuracy of reach of the observer towards an object. An error of spatial localisation allows us to conclude that in PDS there is an error of the perception of the tonicity of the oculomotor muscles. Active Prisms normalise the error of localisation.